The present invention relates to a circuit for controlling the power dissipated by an electrical resistance and particularly to a circuit which does not require an RMS to DC converter.
Electrical resistance heaters are used in a variety of environments. While control of the power dissipated by the electrical resistance heater is not always critical, in certain environments, it is critical that the power dissipated by the electrical resistance heater be controlled so that it does not vary by more than a small percentage.
An example of one type of environment in which the control of the power dissipated by an electrical resistance heater is critical is in a split thermowell in a water induction monitoring system. A water induction monitoring system may be used in a power plant to check for water in the steam lines of a steam turbine. The water induction monitoring system typically includes a plurality of split thermowells which are positioned in the steam lines.
FIG. 1 is a schematic cross-sectional view of an example of a split thermowell 20 which is employed in a water induction monitoring system. Referring to FIG. 1, the split thermowell 20 is mounted in a steam line or pipe 22 and extends approximately 8 inches into the steam pipe. The split thermowell 20 includes a casing 24 having a U-shaped portion 26. Mounted within the casing 24 are an electrical resistance heater 28 and thermocouples 30 and 32. The resistance heater 28 is coupled to a power supply 29 and is used to heat the thermocouples 30 and 32 which in turn provide sensing signals T1 and T2, respectively, to a central control circuit (not shown). As long as there is no water between the legs of the U-shaped portion 26 of the casing 34, the difference between the two temperature signals T1 and T2 output by the thermocouples 30 and 32 should be relatively high (on the order of 10.degree. to 200.degree. F.). However, if the difference between the temperature signals T1 and T2 drops to less than 10.degree. F., then this is an indication that there is water between the legs of the U-shaped portion 26, due to the fact that water has a thermoconductivity which is greater than that for steam. When this drop in the difference in temperature is detected, the water induction monitoring system will issue an alarm.
In order for the split thermowell 20 to function properly, it is necessary that the power of the electrical resistance heater 28 be regulated. The power dissipated by the resistance heater 28 is given by the following equation: EQU P=I.sub.RMS.sup.2 .times.R.sub.H ( 1)
where I.sub.RMS is the RMS current flowing through the heater 28 and R.sub.H is the resistance value of the resistance heater 28. There are typically variations in the supply voltage which is provided to the resistance heater 28. Since the resistance value of the resistance heater 28 does not change, such variations in the supply voltage will result in variations in the current (I.sub.RMS) flowing through the resistance heater 28, with resulting variations in the power (P) dissipated by the electrical resistance heater 28. Thus, the power will tend to vary unless the RMS current of the resistance heater 28 is regulated. The power dissipation in the resistance heater 28 in a split thermowell 20 is controllable to be from 5 to 65 watts based on its position in the system. Typically, the RMS current through the resistance heater is controlled to keep the power dissipated constant.
The RMS current of an electrical resistance heater for pulsed current waveforms (e.g., see FIG. 2) is given by the following equation: ##EQU1## Thus, in order to maintain the RMS current constant, it is necessary to adjust the duty cycle (t.sub.on /T) with variations in the peak current.
FIG. 2 is a waveform diagram illustrating the duty cycle for the current waveform of the current flowing through the resistance heater 28. Thus, the RMS heater current (I.sub.RMS) must be regulated because the peak current will change with line voltage variations which may vary by as much as .+-.10%. If it is assumed that the resistance value of the heater 28 stays the same, then in order to have the power maintained constant, the ON time of the current which flows through the resistance heater 28 must be controlled. For example, the higher the current, the more OFF time that is required. In the prior art, a switching type regulator utilizing pulse width modulation or phase control is commonly used for heater current control to produce a pulsed heater current waveform of the type illustrated in FIG. 2.
As indicated above, there are a number of circuits employing electrical resistance heaters for which control of the power dissipated by the electrical resistance heater is desirable. An example of one circuit which has been employed in the prior art to control the power dissipated by an electrical resistance heater is illustrated in the block diagram of FIG. 3 of the drawings. In FIG. 3, an electrical resistance heater 34 has a resistance R.sub.H and is coupled to a supply voltage V.sub.BUS. While the resistance R.sub.H will tend to remain constant, there will typically be variations in the supply voltage V.sub.BUS. A sensing resistor 36 has a resistance R.sub.S and is employed to sense the current flowing through the resistance heater 34. An RMS to DC converter 38 including a squaring circuit 40, an averaging filter 42 and a square root circuit 44, squares the voltage across the sensing resistor 36, averages the squared voltage and takes the square root of the average, to provide a voltage signal which is proportional to the RMS current flowing through the resistance heater 34. This feedback voltage is then subtracted from a voltage corresponding to the RMS current reference value by a subtractor 46 to provide a current error signal. The current error signal is amplified by an integrator 48 which provides the high steady state loop gain required for precise regulation. The amplified current error signal is applied to a pulse width modulator 50 which generates the required duty cycle for driving a switching device 52 such as a MOSFET, bipolar transistor, etc. When the switching device 52 is ON, current flows through the resistance heater 34. The current which flows through the heater 34 is determined by the supply voltage V.sub.BUS and the resistance values R.sub.H, R.sub.S and the resistance of the switching device 52.
Since the supply voltage V.sub.BUS is unregulated it will tend to vary with the AC supply voltage by .+-.10%. The RMS to DC converter 38 is required because of this DC supply voltage variation. Without the RMS to DC converter 38, the control circuit would act to regulate the average heater current rather than the RMS heater current. As a result, the heater power dissipation would vary by the same percentage as the DC bus voltage variation (i.e., .+-.10%) if the average heater current is maintained constant. By employing the RMS to DC converter 38, the RMS current is controlled to within .+-.1% with line voltage variations of .+-.10%. While RMS to DC converters are available as monolithic integrated circuits, accurate laser trimmed versions of these converters are very expensive, and lower cost versions require undesirable external trimming. Further, in systems with a large number of electrical resistance heaters, it is necessary to provide such a high cost RMS to DC converter for each electrical resistance heater. For example, in the water induction monitoring system described above, there may be as many as fifty thermowells 20, and thus fifty electrical resistance heaters 28 in the monitoring system. Therefore, each electrical resistance heater 28 will require an expensive RMS to DC converter to control the power dissipated by the electrical resistance heater 28.
There is a need in the art for a low cost circuit for accurately controlling the power dissipated by an electrical resistance heater.